The present invention pertains generally to the field of radio frequency (RF) devices and, more specifically, to techniques for second-order harmonic tuning of active RF devices. By way of non-limiting example, the invention relates to RF power amplification circuits in wireless communication devices and networks.
The use of transistor devices as signal amplifiers in wireless communication applications is well known. With the considerable recent growth in the demand for wireless services, such as personal communication services, the operating frequency of wireless networks has increased dramatically and is now well into the gigahertz (GHz) frequencies. At such high frequencies, Gallium Arsenide field effect transistors (GaAs FETs) have been preferred for power amplification applications, since GaAs FETs have a relatively high saturation power efficiency at frequencies of a few giga-hertz, e.g., at 2 GHZ.
RF transistors, such as GaAs FETs are commonly employed in power amplifier circuits in wireless communication devices for amplification of RF signals transmitted at a fundamental frequency f0. Generally, the amplified signal at the output of the RF transistor not only includes the fundamental frequency f0, but also a second-order harmonic frequency 2f0 (xe2x80x9csecond harmonicxe2x80x9d) due to non-linearity in the RF transistor. Using second-order harmonic tuning, the power efficiency of the RF transistor can be improved by suppressing the second harmonic at the output of the RF transistor.
The present invention is directed to providing second harmonic tuning of active devices, such as RF transistors, used in RF amplifier circuits.
An amplifier, built in accordance with one embodiment of the invention, comprises a RF transistor having an output terminal, a quarter wavelength stub, a transmission line and an impedance matching network. The impedance matching network is coupled between the output terminal of the RF transistor and the transmission line. The quarter wavelength Stub provides a short circuit for the second harmonic in the amplified output signal, and is positioned along the transmission line such that the RF transistor is presented with a desired output impedance for tuning the second harmonic.
In one embodiment, the quarter wavelength stub may be coupled to a direct current (DC) voltage bias source for DC biasing to the output of the RF transistor.
In accordance with another aspect of the invention, a test structure is used to determine the position of the xc2xc wavelength stub along the transmission line that results in an optimal (or otherwise desired) impedance at the output of the RF transistor for tuning the second harmonic. The test structure comprises a transmission line and a plurality stubs on the same side of a dielectric layer with a ground plate on the opposite side of the dielectric layer. Each one of the stubs on the test structure is spaced from the transmission line by a gap at different positions along the transmission line. The impedance at the output of the RF transistor for the second harmonic can be measured for each one of the stubs on the test structure by connecting the gap end of the respective stub to the transmission line. The impedance at the output of the RF transistor is measured for different stubs on the test structure until the impedance measurement for one of the stubs approximates optimal or desired impedance.
Other objects and features of the present invention will become apparent from consideration of the following description taken in conjunction with the accompanying drawings.